The present invention relates to a method of detecting moving and/or stationary objects in the path of a vehicle, e.g., for predicting the roadway course or for being able to assign vehicles in front to certain lanes.
For example, it is known from German Published Patent Application No. 42 42 700, now U.S. Pat. No. 5,483,242, that a radar sensor may be mounted on a vehicle for detecting objects located or moving ahead. This radar sensor maybe, for example, a component of an adaptive cruise control (ACC), where information is constantly being processed regarding the distance and speed of the vehicle relative to other vehicles and to the conditions of the road. For example, an FMCW radar (frequency modulated continuous wave radar) such as that known from German Published Patent Application No. 195 30 065, now U.S. Pat. No. 6,037,894, is mounted on the outside in the front area of the motor vehicle and has both transmission technology with an antenna system to generate radar beams and a receiver with an electronic analyzer. According to German Published Patent Application No. 195 30 065, now U.S. Pat. No. 6,037,894, a lateral position determination of objects can also be performed by a multi-beam method, e.g., in the case of motor vehicles turning a curve.
The arrangements mentioned above make use of the Doppler shift which occurs when a reflecting object moves relative to the object to be measured. In order to determine both the distance and speed of the reflecting object, the frequency of the signal emitted is modulated in the shape of a ramp, with the emitted and received signals being mixed during a rise and a fall in the frequency of the signal emitted. The resulting signals are analyzed spectrally, and the distance and speed of the reflecting objects, which are in a distance range that is predetermined by the respective modulation deviation, are determined from the frequencies of the spectral lines.
To rule out the measurement of illusory objects which is possible here, a multi-ramp method is used in the related art mentioned above, e.g., a four-ramp method, where the rising and falling ramps have a lower slope, i.e., ramps three and four have a different slope in comparison with ramps one and two, preferably amounting to half the frequency deviation. The additional analysis of these reflected signals permits an unambiguous determination of the location of objects in the manner described in German Published Patent Application No. 42 42 700.
The course of the roadway can be detected, i.e., the track or course of the vehicles in front can be predicted from the behavior of objects detected by the radar sensor, namely motor vehicles in the present case. However, with the known methods mentioned above, the only targets that can be used are those which are already in a distance range that is predetermined on the basis of the modulation deviation of ramps one and two (approx. 0-120 m in the case of a modulation deviation of approx. 200 kHz). Such a method is described in German Patent No. 197 22 947, for example.
In addition, it is known that a track or course can be predicted with the help of various sensors in the vehicle itself. For example, the yaw rate, i.e., the angular velocity of the vehicle about its vertical axis, which is determined by a suitable sensor, can be analyzed. However, a curve is detected only when the vehicle itself is in the curve, which is too late to make an appropriate prediction of the path. In particular, it is also necessary to prevent regulation with respect to vehicles traveling in neighboring lanes when entering a curve.
A method of detecting moving and/or stationary objects in the path of a motor vehicle, where the distance and the speed of the reflecting object are determined in a multi-ramp method using a radar sensor in the manner mentioned above, is improved in an advantageous manner by analyzing the higher spectral components of a signal from the ramps having a lower slope, e.g., ramps three and four, in the manner according to the present invention. Thus, the upper halves of the spectra, which have not been previously analyzed, from modulation ramps three and four, which detect a distance range from 120 to 240 m, for example, are used for the course prediction with an ACC radar sensor as mentioned above. This permits detection of curves in the course of the roadway before the objects at the beginning of the curve become relevant for the ACC control.
The signals from ramps three and four are to be assigned to a distance range of the reflecting object which is greater than the distance range assigned to ramps one and two because of the lower slope of the ramp, and thus they can also be used extremely advantageously for determining the distance, speed and angle of objects at a greater distance. This information is then available to a suitable vehicle control system at a very early point in time, thus permitting an early decision regarding the relevance of radar targets.
In an advantageous embodiment of the present invention, the respective object angle is determined from the signal peaks obtained from the higher spectral components of the signals of ramps three and four, the signal peaks resulting from the radar beams reflected from an object at a greater distance. Thus, the course of the roadway can be deduced from the time characteristic of the respective object angle, and the track or course can be predicted. The respective object angle can be calculated by way of a multi-beam radar known from German Published Patent Application No. 195 30 065.
When using the method according to the present invention, it is also advantageous to be able to perform a certain discrimination of objects according to their distance and relative velocity. By analogy with the four-ramp analysis method, it is therefore proposed that the points of intersection of two lines in a velocity-distance diagram be calculated in a known manner from the frequencies of the signal peaks of ramps three and four. By permutation of all frequencies from ramp three with all frequencies from ramp four, N2 points of intersection are obtained in the case of N detections, of which N2xe2x88x92N represent illusory targets. To reduce the number of illusory targets, two methods are possible.
First, in correlating the peaks, the respective amplitudes may be used as a criterion. This makes use of the fact that the same target supplies approximately the same levels in two successive ramps following one another at an interval of a few milliseconds. Secondly, the angles for the peaks can be calculated before forming the point of intersection. Peaks belonging to one object have approximately the same angle.
Subsequently, the points of intersection of two lines can be assigned to certain object classes (stationary targets, trucks, passenger vehicles) according to their inherent speeds. Likewise, the variation over time of the centers of gravity of all the lateral distances thus obtained can also be determined by low-pass filtering in an advantageous manner from the radial distance and angle of each object detected, this variation being a measure of the curvature of the roadway for the vehicle.